1. Field of the Invention
The present invention relates to a cathode ray tube, and relates more particularly to a process for suppressing phenomenon of electron beam drift in a cathode ray tube.
2. Description of the Prior Art
FIG. 1 is a schematic sectional view of a conventional cathode ray tube. In a neck portion 1a of an envelope 1 of the cathode ray tube, an electron gun device 2 including three electron guns 2R, 2G, and 2B is provided. A fluorescent screen 3 is provided on the inside of the front portion opposite to the neck portion 1a of the cathode ray tube, and a shadow mask 4 is provided on the inner side thereof.
Three electron beams emitted from the electron gun device 2 are focused by a deflecting coil (not shown) to converge into a point on the fluorescent screen 3 through the shadow mask 4. To assure that the electron beams are thus converged into a point, the electron guns of the electron gun device 2 are tilted slightly while a four-pole magnet ring 5 is rotatably provided around the neck portion 1a for making adjustment, a so-called static convergence adjustment.
FIG. 2 is an enlarged sectional view of the electron gun device 2 shown in FIG. 1 including cathodes 21R, 21G, and 21B, a control electrode 22, an accelerating electrode 23, a focusing electrode 24, and an anode 25. The anode 25 is held at the same potential as the shadow mask 4 and the fluorescent screen 3. The neck portion 1a is made of insulating glass.
At the time of normal operation of the electron gun device 2, the anode 25 is applied with a high voltage of 20 KV with the cathodes 21R, 21G and 21B being applied with zero voltage. Ideally, it is preferred that the insulating inner wall of the neck portion 1a is brought to a state in accordance with such potential gradient as stated above so that the electron beam is influenced by stable electrostatic force from the neck portion 1a.
However, at the time of manufacture of the cathode ray tube, a different voltage from that at the normal operation is sometimes applied to the cathode for the purpose of activation process of the cathode, for example, which causes the potential gradient on the neck portion 1a to gradually vary with time until a stable potential gradient is established therein by virtue of gradual penetration of the anode voltage into the neck portion 1a in the subsequent continued normal operation. Accordingly, electrostatic force from the neck portion 1a which is exerted on the electron beam gradually varies, thereby causing gradual variation in the trajectory of the electron beam. Thus a gradual displacement of the electron beam spot on the fluorescent screen 3 with time, a so-called drift phenomenon, occurs.
The drift amount of the electron beam on the fluorescent screen is about 0.1 to 0.3 mm after 10 to 20 minutes of normal operation. Therefore, it has hitherto been possible to make the drift amount so small as to cause no hindrance in practical use of the cathode ray tube by means of the raster aging for 10 to 20 minutes or by the convergence adjustment with the four-pole magnet at the time of adjustments after manufacture of the cathode ray tube. Recently, however, as external diameter of the neck glass has become smaller, from 36 or 29 mm to 22.5 mm, the stabilizing time for the electron beam drift has become longer.
FIG. 3 shows graphs plotting the electron beam drift amounts measured for 22.5 mm cathode ray tubes. In FIG. 3, a dotted line graph A relates to a conventional cathode ray tube, and solid line graphs B and C relate to cathode ray tubes treated with the process in accordance with the present invention. In FIG. 3, the ordinate represents static convergence drift (in mm) and the abscissa represents the normal operating time (in hours). As apparent from the dotted line A in FIG. 3, the drift amount becomes 1.5 to 2 mm after 1.5 to 2 hours of normal operation and thereafter it is stable. Thus, the cathode ray tubes with smaller neck diameter have been found to have the disadvantage of long stabilizing time.